"Has bailing out the water ever actually gotten the boat dry? Why are you all wasting your time?"
It seems like a simple question that has still gone unanswered. I understand that everyone might be talking about two different things, but in a discussion about a crowd management solution, saying the parks need more capacity misses the point. Genie+ and Lightning Lane (and Fastpass before it) was offered as a guest service to people who demanded the opportunity to wait less in the parks. It's addressing the very real guest complaints that are generated when lines are long in the park. Those guest complaints don't stop just because you build a new attraction. Their focus just changes to the new attraction you are offering that has a really long wait because it's in high demand.
There is a really good example of this at Disneyland, during the Eisner era:
1986: Captain EO opens (3 hour line)
1987: Star Tours opens (3 hour line, but now Capt EO is OK)
1989: Splash Mountain opens (3 hour line, but now Star Tours is an hour)
1992: Fantasmic opens (its a mess)
1993: The riverfront has to be completely redesigned to handle the increased crowds for Fantasmic. It's still a mess.
1994: Cartoon Spin opens (that never had a three hour line)
1995: Indiana Jones opens (3+ hour lines).
1996: Captain EO closes because it's relatively empty.
It's not just a matter of induced demand, where new attractions are generating additional attendance, but also of changing crowd patterns and moving people around the park in unintended ways. Since the time a guest spends in the park is relatively fixed, and they will only ever experience a fraction of the attractions in a park, the ones they dedicate their time and attention to, are usually on the higher end of the tier listing. Changing the attraction roster just changes the individual calculations of each guest every day.
It should also be noted that in the 10 years above at Disneyland, the lowest performing attractions were also pruned from the roster as guests started to shun them: America Sings, Mission to Mars, Circle Vision, People Mover, Motor Boat Cruise, Skyway and Fantasyland Autopia. There just wasn't enough time in the day, to stand in line for Indy and Splash, still do favorites like Pirates and Mansion, and have time for those little ancillary experiences. And anyone that was around Disneyland in the late 1990s knows that there were very real threats that others would be removed too: Lincoln, Tiki Room, Country Bears, Submarine Voyage and Snow White were all suffering to find crowds during this period. It turns out that adding capacity to the top, just drops utilization at the bottom.
This is the period when Fastpass started to be developed. On the heels of ten years of unprecedented growth. In a period where new attractions every year still generated guest complaints regarding crowding and long lines. Where older attractions struggled to find an audience, despite record attendance.
So not, it's not a question of whether bailing out water actually works because we know it doesn't. It's a fundamentally different question entirely. Like thinking that making the boat go faster will drain the water: it won't and it never has.